What is the reason behind hydrogen and helium being the most abundant elements in the universe? originally appeared on Quora, the place to gain and share knowledge, empowering people to learn from others and better understand the world. You can follow Quora on Twitter, Facebook, and Google Plus.
Hydrogen atoms are just protons. Protons are the only stable composite particles that can be formed from quarks. So once the early universe is cold enough for quarks to get into bound states, you end up with lots of protons, i.e., lots of hydrogen.
But not just protons. There is still plenty of excess energy available, so neutrons, which require a little more energy than protons, also form. Free neutrons are not stable: they decay with a half life of about 10 minutes, but that’s almost like an eternity compared to the fact that we are still within the first fraction of a second of the existence of the universe.
Now these protons and neutrons bounce into each other and sometimes they stick to each other. A crude but applicable analogy: take a bunch of hard rubber balls, make them sticky, throw them in a big box. If you shake the box vigorously, the balls will be bouncing about like crazy, sticky or not. But if you only shake the box gently, balls will begin to stick together and stay stuck; sometimes, you’ll see a pair or a triplet of balls bouncing about (gently) as a unit, without coming apart.
Something similar happens when a proton and a neutron stick together to form a deuterium atom; or when two deuterium atoms stick together and form a helium atom. Both these reactions release energy, so the resulting atoms are quite stable; it would take a lot of energy to rip them apart, and there is not that much excess energy available as the universe rapidly cools and atoms are bouncing about more gently than before.
But here, we also run into a problem. Further fusion of, say, a helium atom and a deuterium atom is not possible because the resulting fusion products are highly unstable. That is to say, you need to invest energy into these fusion processes and that energy is almost immediately released when the fusion product breaks down again. Stars overcome this through the famous triple-alpha process, which allows three helium atoms to fuse together to form stable carbon atoms and release energy; but this process is slow, and there is just not enough time in the early universe for it to happen.
So we are stuck with hydrogen, deuterium, helium, and trace amounts of a few other light elements. Everything else in the periodic table has to come later, much later in fact, as heavier elements form in large stars, in supernova explosions, in neutron star mergers and other processes.
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